12,319 research outputs found

    Moduli Spaces and Formal Operads

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    Let overline{M}_{g,n} be the moduli space of stable algebraic curves of genus g with n marked points. With the operations which relate the different moduli spaces identifying marked points, the family (overline{M}_{g,n})_{g,n} is a modular operad of projective smooth Deligne-Mumford stacks, overline{M}. In this paper we prove that the modular operad of singular chains C_*(overline{M};Q) is formal; so it is weakly equivalent to the modular operad of its homology H_*(overline{M};Q). As a consequence, the "up to homotopy" algebras of these two operads are the same. To obtain this result we prove a formality theorem for operads analogous to Deligne-Griffiths-Morgan-Sullivan formality theorem, the existence of minimal models of modular operads, and a characterization of formality for operads which shows that formality is independent of the ground field.Comment: 36 pages (v3: some typographical corrections

    A Cartan-Eilenberg approach to Homotopical Algebra

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    In this paper we propose an approach to homotopical algebra where the basic ingredient is a category with two classes of distinguished morphisms: strong and weak equivalences. These data determine the cofibrant objects by an extension property analogous to the classical lifting property of projective modules. We define a Cartan-Eilenberg category as a category with strong and weak equivalences such that there is an equivalence between its localization with respect to weak equivalences and the localised category of cofibrant objets with respect to strong equivalences. This equivalence allows us to extend the classical theory of derived additive functors to this non additive setting. The main examples include Quillen model categories and functor categories with a triple, in the last case we find examples in which the class of strong equivalences is not determined by a homotopy relation. Among other applications, we prove the existence of filtered minimal models for \emph{cdg} algebras over a zero-characteristic field and we formulate an acyclic models theorem for non additive functors

    Stochastic resonance with weak monochromatic driving: gains above unity induced by high-frequency signals

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    We study the effects of a high-frequency (HF) signal on the response of a noisy bistable system to a low-frequency subthreshold sinusoidal signal. We show that, by conveniently choosing the ratio of the amplitude of the HF signal to its frequency, stochastic resonance gains greater than unity can be measured at the low-frequency value. Thus, the addition of the HF signal can entail an improvement in the detection of weak monochromatic signals. The results are explained in terms of an effective model and illustrated by means of numerical simulations.Comment: 5 pages, 2 figure

    The Economics of Biodiversity Conservation in Agricultural Transition

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    This paper explores the dynamic effects of biodiversity conservation on agricultural production in the context of specialised intensive farming systems that may be in transition towards more sustainable farming. The focus is on the analysis of the dynamic effects of changes in the levels of agrobiodiversity, on technical change and productivity in intensive agricultural systems. A theoretical model is used to derive hypotheses regarding these linkages that are empirically tested using a stochastic production frontier model with data from a panel of UK cereal farms for the period 1989-2000. The results suggest that the increased agrobiodiversity has positively helped to shift the production frontier outwards. This indicates that agricultural transition from more to less intensive agricultural systems can be consistent with non-decreasing output levels and an enhancement of biodiversity in agricultural landscapes.agrobiodiversity, intensive agriculture, productivity, technical change, Environmental Economics and Policy, Q12, Q16, Q24,

    Economics and Biodiversity in Intensively Managed Agro-Ecosystems

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    This paper explores the dynamic effects of biodiversity conservation on agricultural production in the context of specialised intensive farming systems. The focus is on the analysis of the dynamic effects of changes in the levels of agrobiodiversity, on technical change and productivity in intensive agricultural systems. A theoretical model is used to derive hypotheses regarding these linkages that are empirically tested using a stochastic production frontier model with data from a panel of UK cereal farms for the period 1989-2000. The results suggest that the increased agrobiodiversity has positively helped to shift the production frontier outwards. This indicates that the evolution of an intensive agricultural system to less intensive use of inputs can be consistent with non-decreasing output levels and an enhancement of biodiversity in agricultural landscapes.Agrobiodiversity, Intensive Agriculture, Productivity, Technical change, Resource /Energy Economics and Policy, Q12, Q16, Q24,

    Joint optimization of power and data transfer in multiuser MIMO systems

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    We present an approach to solve the nonconvex optimization problem that arises when designing the transmit covariance matrices in multiuser multiple-input multiple-output (MIMO) broadcast networks implementing simultaneous wireless information and power transfer (SWIPT). The MIMO SWIPT problem is formulated as a general multiobjective optimization problem, in which data rates and harvested powers are optimized simultaneously. Two different approaches are applied to reformulate the (nonconvex) multiobjective problem. In the first approach, the transmitter can control the specific amount of power to be harvested by power transfer whereas in the second approach the transmitter can only control the proportion of power to be harvested among the different harvesting users. We solve the resulting formulations using the majorization-minimization (MM) approach. The solution obtained from the MM approach is compared to the classical block-diagonalization (BD) strategy, typically used to solve the nonconvex multiuser MIMO network by forcing no interference among users. Simulation results show that the proposed approach improves over the BD approach both the system sum rate and the power harvested by users. Additionally, the computational times needed for convergence of the proposed methods are much lower than the ones required for classical gradient-based approaches.Peer ReviewedPostprint (author's final draft

    Attractants in purified diets

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    Juvenile Penaeus monodon were reared on purified diets containing different attractants used to gelatinize the cornstarch: plain water, shrimp, mussel, squid or trash fish extract. The highest survival rate was observed in the group given the shrimp attractant, followed by mussel, fish and squid. However growth appeared best in the diet containing mussel extract. Mussel extract apparently can be used to enhance the attractability of purified diets
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